Definition of the extent of participation of host factors in the pathogenesis of bronchiolitis and pneumonia caused by respiratory syncytial virus and parainfluenza virus type 3 is essential to the development of preventive measures against these agents. It has been suggested that immune mechanisms may be involved in the pathogenesis of severe bronchiolitis and, if so, this could restrict the type of immuno-prophylaxis which should be developed. It is proposed to examine these issues by isolating the elements of the host response in experimental infection of hamsters. The effect of various levels of passively-acquired, circulating antibody on infection and reinfection with these viruses will be studied; additionally, the influence of antithymocyte serum will be examined in order to block thymus-dependent factors, thereby isolating the role of passive antibody. Parameters to be measured include serum and surface aatibody responses, quantitation of pulmonary pathology and virus titers, and description of the intraluminal cellular response and ultrastructural changes in the respiratory epithelium. Similar studies also will be done in tracheal organ cultures from immune and non-immune animals. The effects of antibodies and immune cells will be examined in this system. Another important aspect will be examination in hamsters of the factors related to the adverse effect in children of an inactivated RS virus vaccine. Animals will be infected after immunization with aqueous and alum-precipitated vaccines and the parameters of infection will be elaborated as above. Natural reinfections with RS virus and parainfluenza virus type 3 are common and have been associated with exacerbations of asthma and chronic bronchitis. The long term effects of these infections will be studied in hamsters maintained for 1 year or longer. Groups of animals will be sacrificed and examined at intervals for structural and functional changes in the lungs. DTX* 1FR-412-8*